This application claims the priority of 199 24 029.9, filed May 26, 1999, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to the aftertreatment of exhaust gases in internal-combustion engines.
The problems of the aftertreatment of exhaust gases were explained in a comprehensive overview in Bosch/Automotive Handbook, 22nd Edition (ISBN 3-18-419122-2) in the chapter xe2x80x9cExhaust Gases of Spark Ignition Enginesxe2x80x9d (Page 486, and on). During the combustion of the air-fuel mixture in internal-combustion engines, a number of combustion products are formed. In addition to the non-toxic main constituents, such as nitrogen, carbon dioxide and water vapor, the exhaust gas also contains a number of less tolerable secondary constituents, including carbon monoxide (CO), nitrogen monoxide (NO), nitrogen dioxide (NO2), hydrocarbons (HC) and solid matter. Nitrogen monoxide is colorless, odorless and tasteless and, in air, is slowly converted to NO2. In its pure form, NO2 is a pungent toxic gas which, at a higher concentration, can irritate the mucous membranes. (A combined term for NO and NO2 is normally nitrogen oxides NOx.) Hydrocarbons are contained in the exhaust gas in a great variety and, in the presence of nitrogen oxide and sunlight, form oxidants.
This exhaust gas mixture reaches the exhaust gas system where, as a rule, it is first introduced into an oxidation catalyst, which oxidizes the hydrocarbons to carbon dioxide and water and oxidizes the carbon monoxide to carbon dioxide. Simultaneously, nitrogen monoxide NO is also oxidized to nitrogen dioxide.
To reduce harmful fractions in the exhaust gas, a number of measures are known for influencing the exhaust gas composition. These measures can essentially be divided into engine-related measures and measures concerning the aftertreatment of exhaust gases. Since the present invention relates to measures for the aftertreatment, the engine-related measures will not be explained here in detail.
The possibilities for the aftertreatment of exhaust gas include thermal afterburning and catalytic afterburning. The catalysts used for the catalytic afterburning normally consist of a substrate material with an active coating which promotes the chemical conversion particularly of the harmful constituents of the exhaust gas. As an alternative, so-called full-extrusion catalysts may also be used.
Because the efficiency of the catalyst depends essentially on its operating temperature, catalysts tend to be installed close to the engine, which shortens the time required to reach the operating temperature, so that optimal efficiency is rapidly achieved. However, at the same time, installation close to the engine has the disadvantage of a high thermal stressing of the catalyst. Also, in the case of a malfunction of the engine, such as ignition failures in the case of spark ignition engines, because of the conversion of the unburnt fuel taking place in the catalyst, the temperature of the catalyst may rise so high that it is destroyed due to melting of the substrate material. As a rule, this is prevented by the use of reliable and maintenance-free ignition systems.
Oxidation catalysts, which essentially oxidize the carbon monoxide CO and the hydrocarbons HC, are known. They are operated either by a lean engine operation or by an additional air injection with an excess of air or in the leaner-than-stoichiometric diesel exhaust gas, such that carbon monoxide and the HC constituents are oxidized. Furthermore, reduction catalysts are known which reduce the nitrogen oxides in the exhaust gas. The series connection of the oxidation and reduction catalyst with an air injection between the two catalysts has the advantage that NOx as well as HC and CO can be reduced.
The operating principle and the method of operation of a reducing agent catalyst, as well as a method and apparatus for metering a reducing agent into a nitrogen-oxide-containing exhaust gas for reducing these constituents in the reduction catalyst connected on the output side, are known. See, for example, German Patent Document DE 43 15 278 A1.
One object of the invention is to provide a process for monitoring the operation of an exhaust gas treatment system that includes a series connection of an oxidation catalyst and a reduction catalyst, which process can monitor the operation, and thus the reliability, of each catalyst individually or both can be monitored together.
The operation of the oxidation catalyst is permanently impaired by an excessive thermal stressing at exhaust gas temperatures that are too high, which results in a thermal aging. Decisive factors with respect to the extent of the aging are the frequency and the magnitude of the critical temperatures above a reference value. By adding up, or accumulating over time, the temperatures above the reference value, information can be obtained concerning the thermal stressing of the oxidation catalyst. That is, the sum thus formed is compared with a definable limit value; when the sum exceeds the limit, a defect report takes place. The system thus permits very rapid communication to the driver, visually and/or acoustically, concerning the critical condition of the oxidation catalyst. Testing of the reduction catalyst by detecting the nitrogen oxide (NOx) or ammonia (NH3) values in the exhaust gas downstream of the catalyst results in differentiated information concerning the operability of this reduction catalyst or of the reducing agent metering device.
The process according to the invention as described above can be performed by means of respective temperature sensors assigned to the catalysts in the exhaust gas system, as well as sensors for detecting NOx content of the exhaust gas assigned to the reduction catalyst. The provision of two temperature sensors, one arranged upstream of the oxidation catalyst and one downstream of it, has the advantage that, by comparing the temperatures in front of and behind the oxidation catalyst, significantly more precise information can be provided concerning the actual temperature in the oxidation catalyst (oxi-cat). Because each temperature sensor can also be evaluated separately, continued monitoring of the operability of the oxidation catalyst is ensured, even in the event of the failure of a temperature sensor.
The detection of the NOx and/or NH3 concentration in the exhaust gas behind the reducing agent catalyst, and comparison of the NOx concentration at a defined operating point of the internal-combustion engine with a stored control value of the NOx content has the advantage that information can be obtained very rapidly concerning the exhaust gas composition. When an increase of the NOx concentration is detected, a defect report is made to the driver, preferably by a visual and/or acoustic information provided in the instrument cluster. Simultaneously, a corresponding defect entry can take place in the control unit which can then be read out in a repair shop. Here, a detailed information can be stored, as, for example, which catalyst has caused the defect report or the amount of the maximum temperature in the oxidation catalyst or of the NOx or NH3 concentration behind the reduction catalyst.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.